Process for hardening iron, steel, and cast iron alloys by nitriding



Patented Aug. 14, 1934 UNITED STATES PATENT OFFICE PROCESS FOR HARDENINGIRON, STEEL, AND CAST IRON ALLOYS BY NITRIDING No Drawing. Serial No.1930 4 Claims.

The invention relates to processes for hardening iron, steel, and castiron, alloys by nitriding and has for its object to improve theseprocesses in so far as a greater nitride depth is obtained than it washitherto possible, as well as an increased hardness of the nitridedlayer.

It is known that heated metal surfaces such as those of the alloys ofiron, steel and cast iron that are suitable for nitriding, actcatalytically upon ammonia gases contacted therewith therebyaccelerating the dissociation of the ammonia. Similarly, certain of themetals used in constructing containers for nitriding act catalyticallyupon ammonia at nitriding temperatures. At the same time, it has beenfound that better and more uniform nitriding results are obtained whenthe percentage dissociation of the ammonia gas passed over the surfacesintended to be nitrided is kept within certain advantageously within arange of about 30 to 40%. Owing to the catalytic eflect of the heatedmetal surfaces exposed to the ammonia it has been heretofore diflicultto control the percentage dissociation of the ammonia within the desiredrange except by increasing the rate of circulation of the ammoniathrough the nitriding container, and this method has proven relativelyexpensive because of the increased consumption of the ammonia.Furthermore, increase of the rate of circulation of the ammonia does notentirely prevent a higher percentage dissociation of the ammonia in thelocalized areas where the ammonia gas contacts with the hot metalsurfaces of the articles being nitrided.

It has been found that substances which act as catalyst poisons in themanufacture of ammonia also serve to bring about an increase in thehardness and depth of the hardness layer in nitride hardening. Thesesubstances serve to inhibit the catalytic action of the metal surfacesupon the ammonia gas and thereby retard the decomposition of theammonia. In this way control of the rate of decomposition of the ammoniathrough control of the nitriding temperature and the rate of thecirculation of the ammonia is made certain.

Application October 10, 1931, 568,209. In Germany November 1,

The improved results above mentioned may be obtained by carrying out thenitriding in the presence of a metal oxide, such for example as cobaltoxide, which is decomposable under the conditions obtaining in thenitriding operation to yield the catalyst poison. I have also usedcobalt sulphate with similar results.

In carrying the invention into efiect the articles to be hardened may,for example, be sprinkled with a thin layer of pulverized cobalt oxideand then nitrided in the usual way in a steam of ammonia at about 500C., the cobalt oxide being reduced during the nitriding and thedecomposition products acting as catalyst poisons. By this process witha given duration of the nitriding treatment, a greater nitriding depthis produced than with a nitriding treatment of the same duration withoutthe use of a catalyst poison.

What I claim and desire to secure by Letters Patent is:-

1. In the art of hardening ferrous base alloys by nitriding, theimprovement which comprises treating such an alloy with ammonia gas at anitriding temperature in the presence of a substance selected from thegroup consisting of cobalt oxide and cobalt sulphate.

2. In the art of hardening ferrous base alloys by nitriding, theimprovement which consists in nitriding such an alloy while contactingsaid alloy with a substance selected from the group consisting of cobaltoxide and cobalt sulphate.

by nitriding, the improvement which comprises treating such an alloywith ammonia gas at a nitriding temperature while contacting said alloywith finely divided cobalt oxide.

4. In the art of hardening ferrous base alloys by nitriding, theimprovement which comprises treating such an alloy with ammonia gas at anitriding temperature while contacting said alloy with finely dividedcobalt sulphate.

O'I'I'O HENGSTENBERG.

3. In the art of hardening ferrous base alloys 1

